Joseph Dvorak

Fundamental studies of desulfurization processes: reaction of methanethiol on ZnO and Cs/ZnO

The reaction of methanethiol on ZnO and Cs promoted ZnO surfaces has been studied with synchrotron based photoemission and thermal desorption spectroscopy. On ZnO, methanethiol undergoes selective reaction to produce carbon monoxide (37–58%), methane (23–38%), formaldehyde (12–15%), ethane (1–11%), and a mixture of ethylene and acetylene (3–13%). At low temperatures (<100 K), methanethiol reacts to yield thiolate intermediate bound to Zn2+ cations. The thiolate is stable to 500 K. Above this temperature, C–S bond cleavage occurs to yield methyl intermediate and atomic S. Carbon is removed from the surface as gaseous products above 500 K, and atomic sulfur remains bound to the zinc sites of the surface. Submonolayer amounts of cesium do not have a significant promotional effect on C–S bond cleavage, whereas Cs multilayers are found to significantly lower the activation barrier for C–S bond cleavage. This study illustrates the chemistry associated with the desulfurization of thiols on a catalytically relevant oxide surface.

The interaction of oxygen with TiC(001): photoemission and first-principles studies.

High-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of oxygen with a TiC(001) surface. Atomic oxygen is present on the TiC(001) substrate after small doses of O(2) at room temperature. A big positive shift (1.5-1.8 eV) was detected for the C 1s core level. These photoemission studies suggest the existence of strong O<-->C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of O. Ti and C atoms are involved in the adsorption and dissociation of the O(2) molecule. In general, the bond between O and the TiC(001) surface contains a large degree of ionic character. The carbide-->O charge transfer is substantial even at high coverages (>0.5 ML) of oxygen. At 500 K and large doses of O(2), oxidation of the carbide surface occurs with the removal of C and formation of titanium oxides. There is an activation barrier for the exchange of Ti-C and Ti-O bonds which is overcome only by the formation of C-C or C-O bonds on the surface. The mechanism for the removal of a C atom as CO gas involves a minimum of two O adatoms, and three O adatoms are required for the formation of CO(2) gas. Due to the high stability of TiC, an O adatom alone cannot induce the generation of a C vacancy in a flat TiC(001) surface.

Chemistry of SO2 and NO2 on ZnO(0001)-Zn and ZnO powders: changes in reactivity with surface structure and composition

Synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) have been used to study the interaction of SO 2 and NO 2 with ZnO(0001)-Zn and polycrystalline surfaces of zinc oxide (films and powders). Important differences are observed when comparing the chemical behavior of the adsorbates on these oxide surfaces. These differences are in part a consequence of changes in structural properties (flat versus rough surfaces), but in some cases they clearly originate in variations in surface composition (zinc ↔ adsorbate versus oxygen ↔ adsorbate interactions). For example, the Zn-terminated (0001) crystal face of ZnO is much less reactive towards SO 2 than polycrystalline ZnO. On ZnO(0001)-Zn and polycrystalline ZnO, the Zn ↔ SO 2 bonding interactions are weak. Adsorption of SO 2 on Zn sites was seen only at temperatures below 200K. In contrast, the SO 2 molecules react readily with O sites of Ar + sputtered ZnO(0001)-Zn or polycrystalline ZnO forming very stable SO 3 species. Due to its radical nature, adsorbed NO 2 is more chemically active than SO 2 . After dosing nitrogen dioxide to ZnO(0001)-Zn at 100 K, chemisorbed NO 2 and NO 3 coexists on the surface. A partial NO 2 ads → NO 3 ads transformation is observed from 150 to 300 K. The data for the NO 2 /ZnO(0001)-Zn system clearly prove that large quantities of NO 3 can be formed on metal sites of an oxide surface as a consequence of partial decomposition or disproportionation of NO 2 . The routes for the formation of SO 3 and NO 3 on ZnO can be different, but these species have in common a high stability and decompose at temperatures well above 500 K. Thus, ZnO powders can be useful as sorbents in DeSO x and DeNO, operations.

Towards 10 meV resolution: The design of an ultrahigh resolution soft X-ray RIXS spectrometer

We present the optical design of the Centurion soft X-ray resonant inelastic X-ray scattering (RIXS) spectrometer to be located on the SIX beamline at NSLS-II. The spectrometer is designed to reach a resolving power of 100 000 at 1000 eV at its best resolution. It is also designed to have continuously variable 2θ motion over a range of 112° using a custom triple rotating flange. We have analyzed several possible spectrometer designs capable of reaching the target resolution. After careful analysis, we have adopted a Hettrick-Underwood spectrometer design, with an additional plane mirror to maintain a fixed direction for the outgoing beam. The spectrometer can cancel defocus and coma aberrations at all energies, has an erect focal plane, and minimizes mechanical motions of the detector. When the beamline resolution is accounted for, the net spectral resolution will be 14 meV at 1000 eV. This will open up many low energy excitations to study and will expand greatly the power of soft X-ray RIXS.

Macromolecular crystallography beamline X25 at the NSLS

A description of the upgraded beamline X25 at the NSLS, operated by the PXRR and the Photon Sciences Directorate serving the Macromolecular Crystallography community, is presented.

High resolution photoemission studies of sulfur interaction with model catalytic surfaces

Abstract Synchrotron-based photoelectron spectroscopy was used to study sulfur reaction with two model catalytic surfaces: Cu/Ru(0001) and TiO 2 (110). Sulfidation of a strained copper film supported on a Ru substrate proceeds only after chemisorption of at least 0.2 monolayers of sulfur. The induction period was probed by STM to reveal significant restructuring of the metal surface and self-organization of sulfur into several distinct nano-patterns. Large chemical shifts of the S 2p core levels allow a definitive assignment of surface species (atomic vs. sulfidic sulfur). Sulfur reacts with a TiO 2 (110) surface readily from the very early stages of adsorption. We identified at least three types of sulfur species, namely S bonded to Ti rows, vacancies in the bridging oxygen rows, and S associated with the SO x groups.

Intrinsic resolving power of XUV diffraction gratings measured with Fizeau interferometry

We introduce a method for using Fizeau interferometry to measure the intrinsic resolving power of a diffraction grating. This method is more accurate than traditional techniques based on a long-trace profiler (LTP), since it is sensitive to long-distance phase errors not revealed by a d-spacing map. We demonstrate 50,400 resolving power for a mechanically ruled XUV grating from Inprentus, Inc.

Metrology of variable-line-spacing x-ray gratings using the APS Long Trace Profiler

As resolving power targets have increased with each generation of beamlines commissioned in synchrotron radiation facilities worldwide, diffraction gratings are quickly becoming crucial optical components for meeting performance targets. However, the metrology of variable-line-spacing (VLS) gratings for high resolution beamlines is not widespread; in particular, no metrology facility at any US DOE facility is currently equipped to fully characterize such gratings. To begin to address this issue, the Optics Group at the Advanced Photon Source at Argonne, in collaboration with SOLEIL and with support from Brookhaven National Laboratory (BNL), has developed an alternative beam path addition to the Long Trace Profiler (LTP) at Argonne’s Advanced Photon Source. This significantly expands the functionality of the LTP not only to measure mirrors surface slope profile at normal incidence, but also to characterize the groove density of VLS diffraction gratings in the Littrow incidence up to 79°, which covers virtually all diffraction gratings used at synchrotrons in the first order. The LTP light source is a 20mW HeNe laser, which yields enough signal for diffraction measurements to be performed on low angle blazed gratings optimized for soft X-ray wavelengths. We will present the design of the beam path, technical requirements for the optomechanics, and our data analysis procedure. Finally, we discuss challenges still to be overcome and potential limitations with use of the LTP to perform metrology on diffraction gratings.

Dynamic monitoring of grating angle at the National Synchrotron Light Source

We present a dynamic monitoring method and monitoring system of grating angle, referred to as the Precise Angle Monitor (PAM), at U4B, a soft x-ray spherical grating monochromator (SGM) beam line at the National Synchrotron Light Source (NSLS). In an SGM, a photon energy scan is accomplished by rotating the grating angle precisely. After several decades of service, the monochromator at U4B developed instabilities that severely impacted the experimental program. Over several hours, either the spectral shape experienced distortions or the spectral peak shifted. In order to directly monitor the grating motion during scans, the optical head of a portable long trace profiler (PTLTP) was installed on U4B as the PAM. We find that the grating rotational motion is not ideal: (1) the scan steps are not smooth and there are high-frequency step angle errors; (2) there is also a low-frequency angle error; and (3) an unstable thermal expansion produces extra rotational error. Measurements of dynamic monitoring are presented, including grating rotation repeatability and thermal instability. The results illustrate the utility of dynamic monitoring of monochromator motion during actual operation.

Development of the XFP beamline for x-ray footprinting at NSLS-II

For over a decade, synchrotron-based footprinting studies at the NSLS X28C beamline have provided unique insights and approaches for examining the solution-state structures of large macromolecular assemblies, membrane proteins, and soluble proteins, for time-resolved studies of macromolecular dynamics, and most recently for in vivo studies of RNA-protein complexes. The transition from NSLS to NSLS-II has provided the opportunity to create an upgraded facility for the study of increasingly complex systems; progress on the development of the XFP (X-ray Footprinting for In Vitro and In Vivo Structural Studies of Biological Macromolecules) beamline at NSLS-II is presented here. The XFP beamline will utilize a focused 3-pole wiggler source to deliver a high flux density x-ray beam, where dynamics can be studied on the microsecond to millisecond timescales appropriate for probing biological macromolecules while minimizing sample perturbation. The beamline optics and diagnostics enable adaptation of the beam siz...